drm/amd/display: Add function and debugfs to dump DCC_EN bit

[why]
Currently to view the DCC_EN bit the entire DTN log
must be dumped. A compact method to view the DCC_EN
bit is desirable.

[how]
Introduce new debugfs interface that only dumps the
DCC_EN bit.

Example usage:
cat /sys/kernel/debug/dri/0/amdgpu_dm_dcc_en

Signed-off-by: Victor Lu <victorchengchi.lu@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>